What is a party? This article presents the argument that rmal party apparatus is only one part of an extended network of interest groups, media, other advocacy organizations and candidates. The authors have measured a portion of this network in the United States systematically by tracking lists of names transferred between political organizations. Two distinct and polarized networks are revealed, which correspond to a more liberal Democratic group and a more conservative Republican group. Formal party organizations, like the Democratic National Committee and the Republican National Committee, tend to receive information within their respective networks, which suggests that other groups serve to funnel information towards the formal party.